Erythropoietin is now accepted as the hormone controlling red blood cell production. A radioimmunoassay has been developed in this laboratory using pure radioiodine labeled erythropoietin and an anti-erythropoietin-antiserum prepared in rabbits. This assays measures not only normal circulating levels, but also depressed levels seen following physiological conditions known to depress erythropoiesis and increased hormone levels associated with stimuli known to increase erythropoiesis such as bleeding, hypoxia or cobalt administration. This radioimmunoassay shows a significant correlation with several bioassay systems. Using this method, polycythemias of primary origin are distinguishable from those of secondary origin. The clinical significance of this proposal is derived from the diagnostic value of a sensitive radioimmunoassay for blood levels of erythropoietin. Clinical work proposed in this study deals with erythropoietin responses to controlled stimuli which may ultimately be of value diagnosing increased or decreased secretory ability. Fetal hypoxemia in high risk pregnancies can be diagnosed by measuring the hormone levels in the amniotic fluid. In addition to human erythropoietin a variety of animal erythropoietins are measurable with this assay. Animal work proposed is concerned with both the production of erythropoietin and the mechanism of action on its target tissues. The role of this hormone in fetal and neonatal erythropoiesis will be investigated as well as site of production and concentrations as a function of age. Other animal studies are concerned with the effects of other hormones on erythropoietin synthesis and release and with extrarenal sources of erythropoietin in adult animals. The effort to produce antibodies with higher affinity will continue. Monospecific antibodies will be isolated and used in morphological studies of the sites of erythropoietin production. Also studies are proposed relating to the development of a radioreceptor assay for erythropoietin. A radioreceptor assay in conjunction with the radioimmunoassay will give a new dimension to the importance of erythropoietin detection in clinical studies. THe knowledge gained from this investigation will add significantly to our understanding of the biogenesis and mechanism of action of erythropoietin and to the role of this hormone in normal and diseased states in humans.